This article examines the introduction of the medical mask in the late nineteenth century at the intersection of surgery, bacteriology and infection control. During this important episode in the longer history of the medical mask, respiratory protection became a tool of targeted germ control. In 1897, the surgeon Johannes Mikulicz at the University of Breslau (now Wroclaw, Poland), drawing on the bacteriological experiments of his colleague Carl Flügge, used a piece of gauze in front of his nose and mouth as a barrier against microorganisms moving from him to his patients. This article explores the social, cultural and medical contexts of this particular use of the mask, in connection with germ theory and surgeons’ struggle with wound infection. It explores the alignment of the new aseptic surgery with the emerging field of bacteriology in a local milieu that favoured interdisciplinary cooperation. The account also follows the uptake of the mask outside of surgery for other anti-infectious purposes and shows how the new type of anti-infectious mask spread simultaneously in operating rooms as well as in hospitals and sanatoria, and eventually in epidemic contexts.